1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device and, more particularly, to a method of manufacturing a bipolar (BIP) transistor capable of being incorporated with complementary metal oxide semiconductor (CMOS) transistors on a single semiconductor substrate.
2. Description of the Related Art
A semiconductor device having a BIP transistor and CMOS transistors is called a BiCMOS device and presents excellent device characteristics by utilizing an advantage of low power consumption and high integration density of the CMOS transistors and a low noise and a high driving capacity of the BIP transistor. However, a device structure of the BiCMOS device and a manufacturing method of the same are relatively complicated, thereby requiring a relatively large chip area and deteriorating device characteristics.
This will be discussed in detail below with reference to FIG. 2 illustrating a method of manufacturing a BiCMOS device according to the prior art as disclosed in, for example, "Subjects related to Process Integration for Submicron BICMOS Technique" (solid state technology/Japanese edition August 1992).
As shown in FIG. 2A, such a P-type silicon substrate 1 is prepared that has an nMOS section (a), a pMOS section (b), an isolation region section (c) and an BIP section (d). N.sup.+ buried layers 2 are formed in the pMOS, section (b) and the BIP section (d), and P.sup.+ buried layers 3 are formed in the nMOS section (a) and an isolation region section (c).
As shown in FIG. 2B, thereafter, an intrinsic epitaxial layer 4 is grown on the whole surfaces of the silicon substrate 1, followed by selectively forming P-well regions 5 and N-well regions 6 in the layer 4. The respective well regions 5 and 6 are formed in contact with corresponding ones of the burried regions 2 and 3.
Selectively formed in the layer 4 are, as shown in FIG. 2(C), field oxide films 7 in order to separate element regions by a selective thermal oxidation known as LOCOS (local oxidation of silicon) method. A P-type base layer 8 is then selectively formed in the well region 6 serving as a collector region. An N.sup.+ type collector contact region layer 9 is thereafter formed so as to reach the N.sup.+ buried layer 2. Thereafter, a gate oxide film 10 is formed over an entire surface.
The gate oxide film 10 is covered with a polysilicon film 11 as shown in FIG. 2D, followed by an emitter opening 13 is selectively formed to expose a part of the base region 8.
Another polysilicon film 12 is then deposited on the whole surface and N-type impurities are ion-implanted into the film 12, followed by being patterned. As a result, as shown FIG. 2F, gate electrodes 14A for nMOS and pMOS transistors and an emitter electorde 14B for the BIP transistor are formed. Each of the electrodes 14 is composed of two polysilicon films 11 and 12. Side walls 15 of silicon oxide film are thereafter formed on the respective sides of the gate electrodes 14A and emitter electrode 14B, and the oxide layer 10 is then selectively remove by using the electrodes 14 and the side walls 15 as a mask.
Thereafter, N-type impurities and P-type impurities are ion-implanted by using resist layers (not shown) as a mask, and heat treatment is performed to activate the implanted impurity ions. Consequently, N-type source and drain regions 16, P-type source and drain region 17 and a graft base region 18 are formed, as shown in FIG. 2F. At this time, the impurities contained in the polysilicon electrode 14B are also diffused into the base layer 8 to form an N.sup.+ -type emitter layer 19. Silicide layers 20 are then formed on the respective surfaces of the regions 16, 17 and 18 and the surfaces of the respective electrodes 14A and 14B to reduce electrical resistance.
Although not shown, interlayer insulating layers and wiring layer are thereafter formed to complete the device.
Thus, the BiCMOS device according to the prior art employs the buried regions 2 and 3 and the epitaxial layer 4. Each of the buried regions 2 and 3 is formed at a high impurity concentration. Accordingly, each of the buried regions 2 and 3 expands in not only vertical direction but also horizontal direction, so that it reaches and contacts with the adjacent buried region. For this reason, large parasitic capacity is produced to thereby lower an operating speed. If the buried regions 2 and 3 would be formed separately from each other in order to eliminate the parasite capacity, the chip area would be inevitably increased and the integration density would be lowered.
During the epitaxial growth, moreover, the so-called out-diffusion occurs in which some of impurities contained each of the buried regions 2 and 3 are diffused outside and the controllability of the out-diffusion is very difficult, as well known in the art. For this reason, the impurity concentration of the epitaxial layer 4 is largely fractuated from the designed value to thereby change the threshold voltage control of each MOS transistor.
Further, in the prior art, a base layer is formed before a gate oxide film is formed, because contamination from a resist that is a mask for ion implanation to form a base layer exerts a bad influence upon the gate oxide film when the resist is applied directly to the gate oxide film. When oxidation is performed after ion implantation of the base, however, redistribution of a profile and suction of boron into an oxide film are produced, thus a shallow and sharp profile cannot be realized as a result. Since the performance of a BIP depends largely on the base profile, it is difficult to obtain high frequency characteristics by this method.
Against this problem, there is a method of performing base layer formation for a BIP, emitter opening, emitter polysilicon film formation, impurity ion implantation and patterning of emitter electrode after CMOS production process, in short, after patterning of gate electrode. However, there is such a drawback that the production process is further elongated when this technique is adopted. It has been +4 mask against a CMOS in the prior art, but +5 mask in this case.